Improve renderer failure handling

[charlesstaats]

Title: Report a failed 3D render child instead of exiting 0, and add a 3D rendering test

The test is skipped when there is nothing to render with: if asy was built without Vulkan, or the Vulkan loader finds no devices. It then exits 77. ctest reports that as "Skipped" (SKIP_RETURN_CODE 77), and make check prints SKIPPED (reason) and carries on. CI runners without a Vulkan device skip it.

When asy renders a 3D scene in a forked child process, the parent waited for the child but ignored its exit status. If the renderer crashed or exited with an error, asy still exited 0 and simply wrote no image. Two examples: a Boehm GC abort ("Collecting from unknown thread") when gccpp is linked and lavapipe is used, or an uncaught vk::IncompatibleDriverError when no Vulkan driver is present.

• picture.cc: if the render child is killed by a signal or exits non-zero, report an error. asy then exits 1 with a message such as runtime: 3D renderer terminated by signal 6 (Aborted). Interactive viewing, which doesn't wait for the child, is unchanged.
• tests/test_render3d.py: renders a small 3D scene to PNG. The test fails if asy exits non-zero or doesn't write a valid PNG. It runs under both ctest (bundled.asy.render3d) and make check.

On a machine with a Vulkan device, the new test fails with the CMake build on master until the fix that stops linking gccpp outside Windows is merged.

Testing (Debian trixie, lavapipe):

• CMake (linux/release, master, which links gccpp): render3d fails with the error above. With GC_DONT_GC=1, ctest passes 8/8.
• Autotools: make check passes, including render3d.
• No Vulkan driver (VK_ICD_FILENAMES pointed at a nonexistent file): the test is skipped under both ctest and make.
• Python lint: black, isort and pylint pass, using the CI steps from misc-sanity-checks.yml.
• macOS and Windows: not tested.

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